Q:
What are the telescope's mirrors made of which allows them to
deflect X-rays?

A:

The requirement for X-ray reflection is that the X-rays must reflect
at very shallow or grazing angles, like a stone skipping off a
pond. That is why Chandra's mirrors are shaped like barrels. Two
reflections off precisely shaped mirrors are required to make an
image. Chandra's mirror assembly consists of four sets of two
mirrors. The mirrors are coated with iridium, a gold-like metal
that was found to be the best reflector of X-rays over a wide
range of energies.